Thorax:Hollow viscus:Bronchial tree
Bronchial tree * Series of tubular, pipe-like structures that conduct air through their lumens into the alveoli where gas exchange occurs. Airway types * Bronchi ** Larger than 1 mm in diameter ** Taper and branch ** Give rise to non-cartilaginous bronchioles * Bronchioles ** 1 mm or less in diameter ** Most distal bronchiole lined by respiratory epithelium is terminal bronchiole * Terminal bronchioles ** Most distal conducting airway ** Give rise to approximately three generations of respiratory bronchioles * Respiratory bronchioles ** Increasing numbers of alveoli extend from their walls ** Give rise to three generations of alveolar ducts * Alveolar Ducts ** Between respiratory bronchioles and proximal alveoli/alveolar sacs ** Straight tubular spaces bounded entirely by alveoli * Alveoli and Alveolar Sacs ** Small, cup-shaped structures *** Outpouchings of respiratory bronchioles, alveolar ducts, alveolar sacs *** Demarcated by thin walls (septa) ** Adult lungs contain approximately 300 million alveoli Functional and Structural Airway Zones * Conducting zone: for conduction of air only. ** Made up of trachea, bronchi and bronchioles. * Transitional zone ** Function: air conduction and degree of respiration ** Components: respiratory bronchioles and alveolar ducts ** Structure: contains alveoli in the walls so can gas exchange * Respiratory zone ** Function: respiration and gas exchange only ** Components: alveoli and alveolar sacs ** Structure: thin walls and contact with capillary membrane RIGHT MAIN BRONCHUS * Origin anterior to esophagus; courses inferolaterally posterior to right pulmonary artery * Shorter and more vertical than left main bronchus - more prone to foreign body aspiration * Divides into right upper lobe bronchus and bronchus intermedius * Right Upper Lobe Bronchus ** Origin from lateral aspect of right main bronchus at or just below carina; more cephalad than left upper lobe bronchus ** Courses horizontally and laterally (1-2 cm) before branching ** Segments: *** Apical segmental bronchus *** Posterior and anterior segmental bronchi * Bronchus Intermedius ** Origin at level of right upper lobe bronchus ** Courses obliquely (3-4 cm); directly posterior to right pulmonary artery ** Branches into middle lobe and right lower lobe bronchi ** Obstruction may produce combined volume loss/pneumonitis in middle and right lower lobes * Right Middle Lobe Bronchus ** Origin from anterolateral wall of bronchus intermedius; same level as origin of lower lobe bronchus ** Courses anterolaterally, caudally, and obliquely ** Segments *** lateral and medial segmental bronchi * Right Lower Lobe Bronchus ** Superior segmental bronchus originates posteriorly from short proximal portion of right lower lobe bronchus ** Right lower lobe bronchus continues as the common basal trunk before dividing into four basilar segmental bronchi *** Medial, anterior, lateral and posterior basilar segmental bronchi supply the segments of the same name respectively LEFT MAIN BRONCHUS * Origin anterior to esophagus; courses inferolaterally * Longer, more horizontal than right main bronchus; less prone to aspiration * Divides into left upper and left lower lobe bronchi * Left Upper Lobe Bronchus ** Origin from left main bronchus; bifurcates or trifurcates ** Most commonly branches into superior and lingular divisions ** Segments: *** Superior portion divides into apicoposterior and anterior segmental bronchi *** Inferior (lingular) portion bifurcates into superior and inferior segmental bronchi * Left Lower Lobe Bronchus ** Segments: *** Superior segmental bronchus courses posteriorly, at or near level of lingular bronchus *** Left lower lobe bronchus divides into three basilar segmental bronchi - anteromedial, lateral and posterior segmental bronchi. *** Course toward and supply respective lung segments FUNDAMENTAL UNITS OF LUNG STRUCTURE Secondary pulmonary lobule is the functional unit of the lung supplied by a terminal bronchiole and a lobular artery. It is marginated by interlobular septa containing pulmonary veins and lymphatics, * They measure 1 and 2.5 cm across, polyhedral in shape. The fibrous septa is continuous with the peribronchovascular interstitium (axial connective tissue) and pleura (peripheral connective tissue). * Each lobule contains a up to a dozen acini and 30-50 primary pulmonary lobules. * Within the secondary lobule, separating adjacent acini is a much less pronounced network of supporting connective tissue which forms the intralobular septa. Lymphatics: two sets * central: runs with the arteries in the peribronchovascular insterstitium * peripheral: in the interlobular septa and drains to the subpleural plexus Acinus * Portion of lung distal to terminal bronchiole including ** Respiratory bronchioles ** Alveolar ducts ** Alveolar sacs ** Alveoli ** Accompanying vessels and connective tissue * Pulmonary functional unit of gas exchange * Acinar diameter of 6-10 mm Primary Pulmonary Lobule * All alveolar ducts, alveolar sacs, and alveoli distal to the last respiratory bronchiole ** Includes blood vessels, nerves and connective tissue ** 20-25 million primary pulmonary lobules in human lungs VARIANTS OF BRONCHIAL ANATOMY * Anomalous Bronchi ** Arise at lower level than normal in bronchial tree * Supernumerary Bronchi ** Supply same segment of lung as respective normal segmental bronchus * Axillary Bronchus ** Supernumerary airway supplying lateral aspect of right upper lobe * Accessory Tracheal Bronchus ** Syn.: Pig bronchus ** Rare; 1-2% prevalence in adults ** Upper lobe bronchus or segmental bronchus; arises from right lateral tracheal wall ** May be occluded by endotracheal intubation; possible resultant infection * Accessory Cardiac Bronchus ** Rare supernumerary bronchus; 0.5% prevalence in adults ** Arises from medial aspect of right main bronchus or bronchus intermedius ** Courses caudally toward mediastinum and heart (hence "cardiac" designation); typically blind-ending * Accessory Oesophageal Bronchus ** bronchus arises directly from the oesophagus * Situs Abnormalities ** Bilateral right-sided airway anatomy; associated with asplenia, congenital heart disease; rare in adults ** Bilateral left-sided airway anatomy; isolated finding or associated with hypogenetic lung syndrome, less commonly with polysplenia